In recent years, there have developed many uses for hollow microspheres of uniform diameter, uniform wall thickness and uniform strength. Hollow microspheres have found industrial uses as filler materials and as proppants to increase gas recovery from gas wells. Though there are known methods for producing hollow microspheres the known methods suffer one or more shortcomings including producing very small microspheres, microspheres of random size distribution, microspheres which contain latent liquid, solid or gas blowing agents, and microspheres which have thin wall sections or walls having small gas bubbles dissolved or trapped in the walls. See, for example, Sowman U.S. Pat. No. 4,349,456 (sol gel process), and De Vos et. al. U.S. Pat. No. 4,059,423 (latent blowing gas process). Other methods that avoid these shortcomings generally involve carrying out the microsphere forming step at high or relatively high temperatures. See, for example, L. B. Torobin U.S. Pat. No. 4,303,431 (glass), U.S. Pat. No. 4,303,603 (plastic), and U.S. Pat. No. 4,415,512 (metal).
Prior to the time applicant made the present invention there was no known simple economical method of producing relatively large hollow microspheres or hollow porous microspheres where the microspheres were substantially spherical, of substantially uniform diameter, uniform wall thickness, uniform void content and uniform void distribution and intercommunication of the voids in the walls and uniform strength and where the microspheres could be produced at about ambient temperatures.
Further, the recently developed processes which use a multiplicity of hollow porous glass or porous plastic tubes coated with semipermeable membranes for carrying out selective gas or liquid separation processes suffer several shortcomings. The porous glass and plastic tubes are joined by headers which are difficult to manufacture and seal, and the glass tubes in use frequently break. The processes using plastic tubes are limited in operating temperatures and pressures due to the tendency of the tubes to creep and/or buckle with increasing temperatures. Also, attempts to make hollow glass tubes with interconnected voids in the tube walls by acid etching of a separated glass phase has resulted in excessively weak glass tube walls.
In addition, the recently developed use of bioengineered microbacteria to produce pharmaceutical and chemical products has been hampered by the absence of a large scale process in which a self-sustaining sterile growth environment for the bacteria could be maintained, which at the same time allowed selective permeation of oxygen and nutrients to the bacteria, and selective removal of the waste products and/or bio produced products from the sterile environment. Though numerous pharmaceutical and chemical products have been produced in the laboratory or by small scale in vivo processes there has not been an economical means developed which would allow general large scale handling and processing of the bacteria and sterile environment and the efficient separation and purification of bioproducts produced.